Petroleum-based resins such as polyethyleneterephthalate, nylon, polyolefin or plasticized polyvinyl chloride (PVC) are widely used today for a wide range of applications, for example, a packing material, and the like. However, such petroleum-based resins are not biodegradable, thereby causing environmental pollution, e.g., emission of a large amount of greenhouse gases during waste disposal processes, etc. Recently, due to gradual depletion of petroleum resources, use of biomass-based resins, typically polylactic acid resins, is widely considered as an alternative.
However, polylactic acid resins have unsatisfactory heat and moisture resistance or mechanical properties as compared with petroleum-based resins; thus, there have been limitations of the fields and applications to which they are applicable. Particularly, attempts have been made to use polylactic acid resins as packaging materials such as packaging films, but they have failed due to the poor flexibility of polylactic acid reins.
In order to overcome the problems of polylactic acid resins, it has been suggested that low-molecular weight flexibilizers or plasticizers be added to polylactic acid resins, or plasticizers produced by addition polymerization of polyether-based or aliphatic polyester-based polyol be applied to the polylactic acid resins, and the like.
However, there are few improvements in the flexibility of packaging films comprising polylactic acid resins, which have been produced according to these methods. Moreover, the packaging films exhibit poor stability as the plasticizers bleed out over time, and the films have disadvantages of an increase in haze as well as a decrease in transparency.
Further, a method of preparing block copolymers by introducing polyurethane polyol repeating units to polylactic acid resins has been suggested to redress the above problems (Korean Patent Nos. 1191966, 1191967, 1191968, and 1191961, and Korean Laid-Open Patent Publication Nos. 2012-0068552, 2012-0068550, 2012-0094552, 2012-0086118, and 2012-0086117).
Meanwhile, polylactic acid copolymers comprising a soft segment in which polyester polyol repeating units are linked via urethane bonds, or a resin composition or a film containing same have been already known. However, there are some problems that the film containing the polylactic acid copolymer has low transparency and high haze due to low compatibility between polyester polyol and polylactic acid, and has poor moisture resistance due to a high moisture content of polyester polyol and polylactic acid. Furthermore, since such polylactic acid copolymers have a wide molecular weight distribution and poor melting characteristics, they are not well extruded to prepare films. Also, the film thus produced has unsatisfactory mechanical properties, heat resistance, and anti-blocking property.
Generally, as described above, polylactic acid resins are very vulnerable to moisture due to a hydrolysis reaction caused by moisture contained therein. As a result of the reaction, the copolymer is in part degraded into lactic acid, monomers or oligomers, and thereby causing molecular weight degradation.
Moreover, lactic acid, monomers and oligomers thus produced are volatilized during a molding process, and may give rise to a contamination or corrosion of the equipment, or deteriorate the quality of finished products. Specifically, in case of a sheet production via extrusion molding, the residual lactic acid, monomers, and oligomers are volatilized during the sheet extrusion process, thereby causing variations in thickness. In case of a sheet production via injection molding, a hydrolysis reaction continuously takes place even after the manufacturing process has been completed, depending on the environment in which the product is used, thereby causing deterioration in its mechanical properties.
Accordingly, there is a continuous demand for a polylactic acid resin composition having improved flexibility and good moisture resistance, as well as superior general properties such as mechanical properties, transparency, heat resistance, an anti-blocking property, or bleedout resistance, which can be effectively used as a polylactic acid resin-based packaging material.